Depends on what kind of show/manga it is, but for the most part, I appreciate anime's/manga's that have older cast of characters a lot more. Shows like Cowboy Bebop, Monster, Trigun, and One Punch Man I find a lot more appealing just by the fact that's it not about a bunch of high school kids and their drama about growing up.

As long as the MC is not stereotyped into their particular character because of age It makes no diffirrence to me. I also wish that there were more anime's with females as the main character. I recently watched "Rideback" where the MC is female and found it refreshing.